1. Field of the Invention
The present invention relates to a Faraday rotator constituting an optical isolator used as an optical countermeasure against the reflection return beam from a high output laser used for optical communication and processing.
2. Description of Related Art
When a beam reflected on an optical surface and a processing surface of an outer portion of a laser resonator returns to a laser element in a semiconductor laser utilized for optical communication, a solid laser utilized for laser processing, and the like, the laser oscillation becomes unstable. When the laser oscillation becomes unstable, signal noise occurs in the case of the optical communication, and the breakage of the laser element in the case of the processing laser in some cases. Therefore, in order to intercept such a reflection return beam so that such a beam does not return to the laser element, an optical isolator is used. The optical isolator is usually made of a Faraday rotator, a polarizer, an analyzer and a permanent magnet.
The terbium gallium garnet crystals (which will hereinafter be referred to as TGG) and terbium aluminum garnet crystals (which will hereinafter be referred to as TAG) have been used as related art Faraday rotators for an optical isolator for a high output laser.
However, TGG and TAG have a small Faraday rotation coefficient per unit length. Therefore, in order to obtain a 45 degree angle of polarization for the purpose of using these crystals as optical isolators, it is necessary that an optical path length be increased. In practice, crystals having a length of as large as around 6 cm had to be used. In order to obtain a high optical isolation, a uniformly large magnetic field has to be exerted on the crystals, so a strong and large magnet was used. This caused the dimensions of the optical isolator to increase. Since the optical path length is large, the shape of a laser beam is deteriorated in the crystals in some cases, so that some kind of optics is needed for correcting the beam. Furthermore, since TGG is expensive, a small-sized, inexpensive Faraday rotator was demanded.
The Faraday rotation coefficient per unit length of a bismuth substituted type rare earth metal iron garnet crystal film (which will hereinafter be referred to as RIG film) used exclusively in the field of optical communication is noticeably large as compared with those of TGG and TAG, so that the optical isolator can be miniaturized greatly. However, it is known that, when the wavelength of light used by RIG becomes short to a level in the vicinity of 1.1 μm used for a processing laser, the light absorption of iron ions becomes high, and that the performance of the optical isolator is deteriorated due to a temperature rise caused by this optical absorption.
As a method of solving the problem of the temperature rise of the RIG film, the invention disclosed in JP-A-2000-66160 was proposed. The techniques disclosed in this document are the techniques for rendering it easy to radiate the heat occurring in the RIG film by leaving a (GdCa)3 (GaMgZr)5O12 substrate (which will hereinafter be referred to as GGG substrate) for growing the RIG film, which is usually removed rather than by being polished. Also, a method of holding both surfaces of a RIG film by transparent garnet substrates and radiating the heat of the RIG film via the garnet substrates of a high thermal conductivity is disclosed in JP-A-7-281129.
In the method disclosed in JP-A-2000-66160, the generation of heat due to the optical absorption in the RIG film becomes not lost, strain caused by the different thermal expansion coefficients of RIG film and GGG substrate combined with each other directly in one body occurs. As a result, birefringence occurs both in the RIG film and GGG substrate, so that, when the RIG film and GGG substrate are used for an optical isolator, the isolation function is deteriorated.
Unless the RIG film is polished by controlling the thickness thereof so that the Faraday rotation angle of the RIG film becomes accurately 45 degrees, the reflected light on an interface between the RIG film and GGG substrate cannot be removed completely by an incident side polarizer, and this reflected light returns to the laser element.
In the invention disclosed in JP-A-7-281129, a method is introduced of bringing a transparent substrate, such as a garnet substrate or a glass substrate into contact with one side or both sides of a RIG film or bonding the same transparent substrate thereto with an optical bonding glue. In the method of merely bringing the RIG film and transparent film into contact with each other, it is difficult to bring the RIG film and transparent film with each other uniformly in the whole of the region of the diameter of the incident laser beam, and an air layer necessarily exists on a non-contacting portion. In this case, an anti-reflection coating formed on the premise that the RIG film and transparent substrate contact each other does not function due to the air layer penetrating therebetween. As a result, large reflected light is made to occur to cause the reflected light to return to the laser element. Moreover, the heat radiating function lower.
When a laser beam having an output level which exceeds 1 W is put into the RIG film even in the method of bonding the RIG film and transparent substrate with an optical bonding glue, the temperature of the RIG film increases, and the Faraday rotation angle of the RIG film deviates from 45 degrees due to the temperature coefficient thereof, and the isolation function is deteriorated.